The manufacture of integrated circuits (ICs) is concentrated not only on memory circuits but also on the production of application-specific integrated circuits (ASICs). A cost-effective and high-quality manufacturing method for ICs requires a consistently flexible and automatic wafer production procedure that can be reliably managed, in particular, with the aid of a process monitoring and process control or regulation system. Particular emphasis is placed here on so-called defect inspection, i.e. the inspection of ICs to determine whether defects have occurred in the individual circuits during production. For this inspection, a suitable method must be selected from a series of possible methods. In the context of computerized or automatic methods, high-performance automatic defect detection systems based on image-image or image-data comparisons are especially suitable.
One such method is known, for example, from U.S. Pat. No. 5,153,444. On a wafer on which a plurality of identical IC circuits are applied, a defect is detected by comparing images of the individual ICs with one another. This is done by firstly acquiring a grayscale image of an IC. This grayscale image is then compared with the grayscale image of an adjacent IC. If the comparison reveals a point at which no conformity exists, this is regarded as a defect. Defect classification, however, requires a further examination of the two ICs, which must be performed at a second workstation.
While these systems are very accurate, they have the disadvantage that throughput times for defect detection are very long, several hours often being required for each wafer. Exact positioning of the individual images of the ICs with respect to one another is furthermore an absolute prerequisite for the reliability of the method. The acquisition and operating costs of such systems are also very high.
Defect inspection can also, on the other hand, be accomplished visually using inspection microscopes. Here, however, operating personnel are exposed to considerable physical stress. The inspection is moreover very time-intensive and error-prone. Scattered-light device have therefore been in use for some time for wafer inspection.
EP 0 524 348 discloses a scattered-light device of this kind. What is exploited here is the fact that very dense and fine defect structures of the surface defects generate not only scattered light but also a certain proportion of diffracted light, since the defect structures act as gratings. A light cone created in this fashion does not possess a local homogeneous intensity distribution. The defect structure can thus be identified using an optical arrangement in which an astigmatic lens system is arranged between the light source and the objective. This system generates a cigar-shaped intermediate image that is imaged by the objective onto the surface. A dark-field stop assembly arranged in the beam path between the lens system and the objective allows a direction-dependent measurement of the intensity produced by the defect structure, so that the latter can be detected. This embodiment of a scattered-light examination system for the inspection of wafers is very productive, but has only poor local resolution. In addition, the identified defect is difficult to distinguish from the background.
The wafer can also be examined pixel by pixel. Here, as proposed in WO 00/02037, a beam is directed vertically onto the wafer surface. The scattered radiation produced thereby at the beam incidence point is sensed using radially arranged detectors, and evaluated for each irradiation point, i.e. pixel by pixel, as to whether characteristics are present that indicate a pixel having defects or a defect-free pixel. This type of surface examination is, however, very memory-intensive and requires a great deal of time.
WO 99/14575 therefore proposes a refined method for scattered-light examination of surfaces in order to detect defect structures. Here the object to be examined is illuminated with a beam that is incident vertically onto the object, and simultaneously with a beam that is directed onto the object with a raking incidence. The two beams are polarized perpendicularly to one another. The scattered radiation generated by the respective incident beam is sensed by a separate detector. Better defect selectivity is thereby obtained, and can be even further improved by the additional application of image-processing methods.
U.S. Pat. No. 5,859,698 likewise discloses a method that allows the detection of defects by scattered-light examination. An automatic image processing system, which compares the image of a sample with a reference image, is used here. The resulting difference image can optionally be further evaluated using additional electronic methods, including morphological transformations or definition of a threshold value. The purpose of these electronic evaluations is to ascertain whether the data obtained from the difference image actually originated from a macrodefect.
In addition to reliable and automatic detection of defects, the defects that are detected also need to be classified. A method and a system for automatic defect classification (ADC) are known for this purpose from WO 99/67626. Here a small region of a wafer is illuminated with a laser beam. Four equally distributed dark-field detectors are arranged in such a way that their sensing angles overlap, thereby forming so-called detection zones. The scattered radiation sensed by the dark-field detectors is converted into electrical signals and conveyed to an analysis unit. The analysis unit is capable of detecting, from the electrical signals, whether a defect is in fact present. Using stored pattern evaluation methods, the analysis unit can additionally perform a classification of the defect, for example according to its size.
U.S. Pat. No. 5,982,921 proposes an apparatus and a method for defect identification on wafer surfaces. In a first phase of the method, the entire surface of an object is optically examined at relatively high speed. Advantageously, a laser beam is used for this purpose to scan the object. The result is then compared with a reference pattern. If specific points suspected of being defects are identified, those points are then examined more closely at higher resolution in a second phase of the method, to determine whether a defect in fact exists. Two mutually independent examination devices are provided to allow the examination to be performed in the individual phases. After examination of the object at the first device, it is transported to the location of the second examination device.